1. Field of the Invention
The present invention relates to image forming apparatuses, such as copiers and laser beam printers, which form a visible image from an electrostatic latent image on a latent-image bearing member by using a developing device.
2. Description of the Related Art
As shown in FIG. 4, a photosensitive drum 1 and a developing sleeve 12 of an image forming apparatus are disposed with a slight sleeve-and-drum (S-D) gap Gsd constantly maintained therebetween so as to allow for a predetermined developing process. If this S-D gap Gsd fluctuates, image formation defects, such as uneven densities of images, can occur.
In related art, the opposite ends/sides of the developing sleeve are respectively provided with positioning rollers for maintaining a fixed gap between the photosensitive drum and the developing sleeve. Specifically, these rollers have a radius greater than that of the developing sleeve by an amount equivalent to the S-D gap and are disposed in abutment with the photosensitive drum so as to maintain a fixed S-D gap. See Japanese Patent Laid-Open No. H06-067525 for an example.
FIG. 5 illustrates a configuration similar to that disclosed in Japanese Patent Laid-Open No. H06-067525. As shown in FIG. 5, the opposite ends/sides of the developing sleeve 12 rotatably support positioning rollers 20 by means of bearings 24. One end of the developing sleeve 12 is provided with a gear 23 that applies a driving force for rotating the developing sleeve 12. The gear 23 is configured to receive the driving force from the outside of a developing device 3.
The S-D gap Gsd is set by bringing the positioning rollers 20 provided at the opposite ends/sides of the developing sleeve 12 into abutment with the outer peripheral surface of the photosensitive drum 1, whereby the developing sleeve 12 can be disposed while the predetermined S-D gap Gsd is maintained between the developing sleeve 12 and the photosensitive drum 1.
In this case, since the positioning rollers 20 are provided by means of the bearings 24, the positioning rollers 20 rotate together with the photosensitive drum 1, whereas the inner periphery of each of the bearings 24 rotates together with the developing sleeve 12.
In order to place the positioning rollers 20 in abutment with the outer peripheral surface of the photosensitive drum 1, the base ends of springs 18 are first attached to a rear section of a developer container 22. The tips of the springs 18 are then supported by stationary plates 19 provided on a main body of the image forming apparatus, thereby pressing the developing device 3 towards the photosensitive drum 1.
Referring to FIG. 4, if the developing sleeve 12 requires maintenance, for example, the developing device 3 can be detached by first removing the stationary plates 19 and then sliding the developing device 3 along guide members 13 and 14.
With this technique in which the S-D-gap forming rollers (i.e., the positioning rollers 20) are placed in abutment with the photosensitive drum 1, the toner may possibly enter the gap between the positioning rollers 20 and the photosensitive drum 1 depending on the durability or the soiled condition thereof. This can cause vibration or fluctuation of the S-D gap, thus resulting in image defects.
A conceivable configuration for the prevention of such image defects is shown in FIGS. 6 and 7. In the description hereinafter, a direction in which the developing sleeve 12 abuts on the photosensitive drum 1 will be referred to as an “S-D direction”. In this configuration, S-D-direction abutment blocks 16 that receive force acting in the S-D direction are provided concentrically with the photosensitive drum 1.
However, with regard to configurations that are made to press the developing device 3 towards the photosensitive member (i.e., the photosensitive drum 1), as shown in FIG. 6, such as the above-described configurations of the related art in which the S-D-gap forming rollers (i.e., the positioning rollers 20) are placed directly in abutment with the photosensitive drum 1 or are placed in abutment with the S-D-direction abutment blocks 16, the following problem exists. Specifically, since the developing device 3 and the guide members 13 and 14 have a clearance therebetween for a dimensional tolerance, there is play therebetween. For this reason, it is difficult to properly set the developing device 3 in position, as shown in FIGS. 10 and 11, thus making it difficult to set the S-D-gap forming rollers in position. In consequence, the S-D-gap forming rollers vibrate unfavorably due to the play between the developing device 3 and the guide members 13 and 14, thus making it difficult to maintain the S-D gap.